1. Field of the Invention
The present invention relates to a zoom lens device for use with a video camera of a single-unit video camera-recorder, for example.
2. Description of the Related Art
As zoom lens devices for use with a video camera of a single-unit video camera-recorder, for example, there is available a so-called compensator-less inner-focus lens.
FIG. 1 of the accompanying drawings shows an arrangement of a zoom lens device including a compensator-less inner-focus lens.
In FIG. 1, reference numeral 1 designates a fixed front component. A so-called variator lens 2 for determining a zooming position is disposed behind the fixed front component 1. A focusing lens 5 for properly focusing an object is disposed behind the variator lens 2 succeeded by an iris mechanism 3 and a fixed lens 4. Reference numeral 6 designates an imaging device, i.e., CCD (charge-coupled device).
In the above-mentioned zoom lens device, when the position of the variator lens 2 is moved straight between A (wide-angle lens mode) and B (telephoto lens mode) for effecting so-called zooming, the position of the focusing lens 5 is moved along loci of predetermined curves 30 and 31 in FIG. 1. Thus, the position of the focusing lens 5 is determined so as to properly focus the object constantly when the cameraman takes a picture of the object away from the camera by an arbitrary distance by properly placing the variator lens 2 at an arbitrary position between the wide-angle lens mode (A) and the telephoto lens mode (B). The curve 31 indicates the case where the focusing lens 5 is placed near the object, and the curve 31 indicates the case where the focusing lens 5 is distant from the object.
To determine the position of the focusing lens 5 along the loci of the curves 30 and 31, a microcomputer is used (not shown) including a memory in which data (cam-operation data) of these curves 30, 31 are stored. Data indicating the position of the focusing lens 5 relative to information indicating the position of the variator lens 2 is read out from the memory of the microcomputer. Moreover, data indicating curves relative to several distances composed of distances from the camera to the object, such as shortest distance to longest distance (infinite) are stored in the memory of the microcomputer. For these distances, the microcomputer proportionally divides data indicating several distances to determine the position of the focusing lens 5. Among the related art, are U.S. Pat. Nos. 5,212,593 and 5,352,882 of the same assignee of the present application.
When the above-mentioned zoom lens device is used by the video camera of the single-unit video camera-recorder, if the zoom lens device is mounted within a closed outer casing of the video camera, there is then the risk that a lens barrel and a lens itself are heated due to a source of high temperature because the temperature within the video camera rises due to heat generated from a base plate or the like.
On the other hand, in the zoom lens device, when a temperature change occurs in the circumstance under which the zoom lens device is used, it is frequently observed that a lens focusing position (focusing position) is displaced depending on temperature characteristics of assembly parts which are affected by environment temperature, such as expansion and contraction of a lens barrel material and a change of curvature of the lens itself.
Specifically, in the above-mentioned zoom lens device, when the object is distant from the camera by an arbitrary distance, let it be assumed that a curve along which the position of the focusing lens 5 (vertical axis) is moved relative to the position of the variator lens 2 (horizontal axis) is indicated by a curve a in FIG. 2. Data indicative of the curve a is obtained under arbitrary environment temperature when the zoom lens device is designed and manufactured, and stored in the above-mentioned memory of the microcomputer.
If, on the other hand, the lens barrel material is expanded or contracted and the curvature of the lens itself is changed as the environment temperature is changed, then the curve a is displaced in the horizontal direction to present a curve b in FIG. 2. In this case, if the position of the focusing lens 5 is moved relative to the arbitrary object under the condition that the variator lens 2 is placed at the telephoto lens end, then the arbitrary object is properly focused at the position B in FIG. 2.
If the variator lens 2 were moved from the telephoto lens end to the wide-angle lens side under the condition that the focusing lens 5 is placed at the position B, the position of the focusing lens 5 would have to be shifted as shown by the curve b. However, in the above-mentioned example, the microcomputer proportionally divides data indicative of the curve a obtained in the arbitrary object distance stored in the memory and data indicating a curve a of the next object distance with the result that the position of the focusing lens 5 is shifted as shown by a resultant curve d.
As a result, due to a displacement of vertical axes of the curves d and b, the object of the arbitrary distance is not properly focused. Therefore, when the position (zooming position) of the variator lens 2 is moved from the telephoto lens end to the wide-angle lens side, the object placed with the arbitrary distance object is not properly focused due to the movement of the position of the variator lens 2. When the object is not properly focused, performance of the zoom lens device is lowered. Further, when the zoom lens device is used together with an automatic focus adjustment device, the automatic focus adjustment device has to adjust the displacement of focus instead of the zoom lens device.
Therefore, the focus displacement caused when the object is not properly focused due to the change of the environment temperature has to be corrected. In the zoom lens device, however, the curve a is not only parallelly fluctuated in the vertical and horizontal directions but also is fluctuated variously depending on lens characteristics of individual lenses, such as expansion and contraction of curvature, structure of the lens barrel and power of the lens or the like. Thus, it is very difficult to predict the fluctuation of the curve a, Further, in order to correct the curve a to obtain the curve b, a complex processing is required and such complex processing cannot be realized with ease.
On the other hand, in the video camera of the single-unit video camera-recorder, as a technology for mounting camera assemblies on the video camera with high density is advanced, the video camera of the single-unit video camera-recorder is miniaturized, becomes handier and can be used in a variety of circumstances. Furthermore, performance which the user requests that the machine obtain becomes high. For example, it is requested that the object can be properly focused under any circumstances in which the video camera is used.